What Is Power Factor And Why Should I Care?

First lets examine what power factor is all about.

Power Factor is a quantity used to describe the power loss on an AC electrical circuit explained as follows.

The power company delivers power to you which is called true power and expressed as (W).  Like buying a soda at your favorite fountain, you want a full glass of soda or in this case, you want to use all the electricity supplied to you.

Unfortunately – AC loads contain many factors affecting impedance on the circuit. Using the above example of the glass of soda, inductive reactance is a condition in all AC circuits which consumes the true power (W) at varying degrees. Most electric motors for example have been designed to operate with a power factor of from 80 to 90 percent at full load.  Therefore your glass is only 80 to 90 percent full.  The useful current (I) consumed multiplied by the voltage (E) supplied is the apparent power of your motor.

The difference between the apparent power (EI) of your motor and the true power (W) is power you are paying for and realizing no benefit.
PF = W/EI

Given the above facts knowing you are only getting 80 cents worth of good out of every dollar you do care Right?

Well lets look at what adding capacitors to your motor circuits can do for you.

ENERGY CONSERVATION

BETTER UTILIZATION OF POWER

REDUCING UTILITY CHARGES

RELEASE OF SYSTEM CAPACITY

ACTUAL MONETARY SAVINGS

IMPROVED VOLTAGE

REDUCE POWER LOSSES

 Caution:  Do not install a capacitor that is larger than what we recommend in the following tables or you will have a leading power factor which can cause big big problems.  Don't Do It!  See * in the following information.

It is like paying for the soda and dropping the glass on the floor, when the power factor goes beyond 100 percent.

READ THIS BECAUSE...
PROPER POWER FACTOR CORRECTION CAPACITOR SELECTION IS CRITICAL.
 
APPLYING CAPACITOR UNITS
KVAR UNITS - INDIVIDUAL MOTORS
Power Factor Correction Capacitors can be applied
There are four different methods of selecting the
at individual motors, distribution panels, or on the
proper size KVAR units for induction motors.
main service panel. Fixed Capacitors can be connected
Choose from one of these methods below based on
at all three locations, or Automatic Capacitor
what information you have available.
Systems can be installed on the main service panel.
 
Fixed Capacitors are permanent values of KVAR connected to the electrical system, while Automatic
1) Use actual load measurements of KW and Power
  Factor. This information can be used with Table
Capacitor Systems vary the amount of KVAR that is
3 below to calculate the KVAR necessary for a desired
connected based on sensing the entire electrical
Power Factor.
system requirements. If plant loads vary widely
 
during any 24 hour period, large fixed capacitors at
2) Use motor manufacturer's recommendations.
the main service panel are not recommended.
Some motors are supplied with maximum
* Overcorrection may result, causing potential
KVAR recommendations.
problems to the capacitors and adjacent connected
 
equipment. In this case individual motor correction
3) Use motor data supplied by motor manufacturer,
or an Automatic Capacitor System would be the best
when full load power factor and full load amps are
installation.
known. First, multiply full load amps times
voltage times 1.723 and divide by 1000. This result
DETERMINE TOTAL REQUIREMENT
is KVA. Next, multiply the KVA times the power
In order to determine the overall KVAR requirements,
factor. This result is the KW. Next, use Table 3 below to
the normal load KW and the original power
determine the KVAR required to raise the full
factor must be known. This information can usually
load Power Factor to the desired level.
be obtained from the electric utility bill or from the
 
local power company. To compute the total KVAR
4) Use Tables 1 & 2 below which list the recommended
required, refer to Table 3 below and multiply the value
sizes of KVAR units needed for correction of
found at the intersection of “Original Power
most induction motors to approximately 95%
Factor” and “Desired Power Factor” by the normal
power factor. These tables show the proper
load KW. As an example: To improve the power
KVAR for a given horsepower and RPM. 
factor of a 400 KW load from .77 to .92 :
 
KVAR = KW x Multiplier
Caution:
= 400 x .403 (Table 3)
Certain motor applications are not suitable for
= 161.2
connecting the capacitor to the load side of the motor
In the above example, 161.2 KVAR would be
starter. Applications involving reversing, plugging,
required to correct the complete system. If individual
or frequent starts; crane or elevator motors, or any
motors are being corrected, the KVAR being
motor where the load may drive the motor, multispeed
connected to individual motors is subtracted from
motors, or motors using open transition
the overall KVAR required for the entire system.
reduced voltage starting, must be corrected on the
The balance would then be connected to the
distribution panel or main service panel.
distribution system. If the total KVAR required was
 
161.2 as noted in the above example, and 100
KVAR had been connected to individual motors, the
balance of 61.2 would be reduced to 60 KVAR and
connected to the distribution system. As an alternative,
a VAR MANAGER 180 KVAR unit could be
used on the main service to correct the whole facility
 

 

TABLE 1
Suggested Capacitor Ratings for T-Frame NEMA Class B Motors
Nominal Motor Speed
  3600 RPM 1800 RPM 1200 RPM 900 RPM 720 RPM 600 RPM
Induction Capa- Line Capa- Line Capa- Line Capa- Line Capa- Line Capa- Line
Motor citor Current citor Current citor Current citor Current citor Current citor Current
Rating Rating Reduced Rating Reduced Rating Reduced Razing Reduced Rating Reduced Rating Reduced
(HP) (KVAR) (%) (KVAR) (%) (KVAR) (%) (KVAR) (%) (KVAR) (%) (KVAR) (%)
3 1.5 14 15 23 2.5 28 3 38 3 40 4 40
5 2 14 25 22 3 26 4 31 4 40 5 40
7.5 2.5 14 3 20 4 21 5 28 5 38 6 45
10 4 14 4 18 ' 21 6 27 7.5 36 8 38
15 5 12 5 18 6 20 75 24 8 32 10 34
20 6 12 6 17 5 19 9 23 10 29 12 30
25 7.5 12 7.5 17 8 19 10 23 12 25 18 30
30 8 11 8 16 10 19 14 22 15 24 225 30
40 12 12 13 15 16 19 18 21 22.5 24 25 30
50 15 12 18 15 20 19 225 21 24 24 30 30
60 18 12 21 14 22.5 17 26 20 30 22 35 28
75 20 12 23 14 25 15 28 17 33 14 40 19
100 12.5 11 30 14 30 12 35 16 40 15 45 17
125 25 10 36 12 35 12 42 14 45 15 50 17
150 30 10 42 12 40 12 52.5 14 52.5 14 60 17
200 35 10 50 11 50 10 65 13 68 13 90 17
250 40 11 60 10 62.5 10 82 13 87.5 13 100 17
300 45 11 68 10 75 12 100 14 100 13 120 17
350 50 12 75 8 90 12 120 13 120 13 135 15
400 75 10 80 8 100 12 130 13 140 13 150 15
450 80 8 90 8 120 10 140 12 160 14 160 15
500 100 8 120 9 150 12 160 12 180 13 180 15

 
TABLE 2
Suggested Capacitor Ratings for Non T-Frame NEMA Class B Open Squirrel-Cage Motors
Nominal Motor Speed
  3600 RPM 1800 RPM 1200 RPM 900 RPM 720 RPM 600 RPM
Induction Capa- Lure Capa- Line Capa- Lure Capa- Line Capa- Line Capa- Line
Motor citor Current citor Current citor Current citor Current citor Current citor Current
Rating Rating Reduced Rating Reduced Rating Reduced Rating Reduced Rating Reduced Rating Reduced
(HP) (KVAR) (%) (KVAR) (%) (KVAR) (%) (KVAR) (%) (KVAR) (%) (KVAR) (%)
3 1.5 14 1.5 15 1.5 20 2 27 15 35 3.5 41
5 2 12 2 13 2 17 3 25 4 32 4.5 37
7.5 2.5 11 2.5 12 3 15 4 22 5_5 30 6 34
10 3 10 3 11 35 14 5 21 65 27 7.5 31
15 4 9 4 10 5 13 6.5 18 8 23 9.5 27
20 5 9 5 10 6.5 12 7.5 16 9 21 12 25
25 6 9 6 10 7.5 11 9 15 11 20 14 23
30 7 8 7 9 9 11 10 14 12 18 16 11
40 9 8 9 9 11 10 12 13 15 16 20 20
50 12 8 11 9 13 10 15 12 19 15 24 19
60 14 8 14 8 15 10 18 11 22 15 27 19
75 17 8 16 8 18 10 21 10 26 14 32.5 18
100 22 8 21 8 25 9 27 10 32.5 13 40 17
125 27 8 26 8 30 9 32.5 10 40 13 47.5 16
150 32.5 8 30 8 35 9 37.5 10 47.5 12 52.5 15
200 40 8 37.5 8 415 9 47.5 10 60 12 65 14
250 50 8 45 7 52.5 8 57.5 9 70 11 77.5 13
300 57.5 8 52.5 7 60 8 65 9 80 11 87.5 12
350 65 8 60 7 67.5 8 75 9 87.5 10 95 11
400 70 8 65 6 75 8 85 9 95 10 105 11
450 75 8 67.5 6 80 8 92.5 9 100 9 110 11
500 77.5 8 72.5 6 815 8 97.5 9 107.5 9 115 10
                       
Tables I & 2 apply so three phase. 60 Hz motors when switched with capacitors as a single unit. IEEE Std 141-1986

 
TABLE 3
KW Multipliers for Determining Capacitor Kilovars
Original Desired Power Factor
Power 0.81 0.82 0.83 0.84 0.85 0.86 0.87 0.88 0.89 0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1
Factor                                        
0.51 0.962 0.989 1.015 1.041 1.067 1.094 1.12 1.147 1.175 1.203 1.231 1.261 1.292 1.324 1.358 1.395 1.436 1.484 1.544 1.687
0.52 0.919 0.945 0.971 0.997 1.023 1.05 1.076 1.103 1.131 1.159 1.187 1.217 1.248 1.28 1.314 1.351 1.392 1.44 1.5 1.643
0.53 0.876 0.902 0.928 0.954 0.98 1.007 1.033 1.06 1.088 1.116 1.144 1.174 1.205 1.237 1.271 1.308 1.349 1.397 1.457 1.6
0.54 0.835 0.861 0.887 0.913 0.939 0.966 0.992 1.019 1.047 1.075 1.103 1.133 1.164 1.196 1.23 1.267 1.308 1.356 1.416 1559
0.55 0.795 0.821 0.847 0.873 0.899 0.926 0.952 0.979 1.007 1.035 1.063 1.093 1.124 1.156 1.19 1.227 1.268 1.316 1.377 1.519
0.56 0.756 0.782 0.808 0.834 0.86 0.887 0.913 0.94 0.968 0.996 1.024 1.054 1.085 1.117 1.151 1.188 1.229 1.277 1.338 1.48
0.57 0.718 0.744 0.77 0.796 0.822 0.849 0.875 0.902 0.93 0.958 0.986 1.016 1.047 1.079 1.113 1.15 1.191 1.239 1.3 1.442
0.58 0.681 0.707 0.733 0.759 0.785 0.812 0.838 0.865 0.893 0.921 0.949 0.979 1.01 1.042 1.076 1.113 1.154 1.202 1.263 1.405
0.59 0.645 0.671 0.697 0.723 0.749 0.776 0.802 0.829 0.857 0.885 0.913 0.943 0.974 1.006 1.04 1.077 1.118 1.166 1.226 1.368
0.6 0.609 0.635 0.661 0.687 0.713 0.74 0.766 0.793 0.821 0.849 0.877 0.907 0.938 0.97 1.004 1.041 1.082 1.13 1.192 1.334
0.61 0.575 0.601 0.627 0.653 0.679 0.706 0.732 0.759 0.787 0.815 0.843 0.873 0.904 0.936 0.97 1.007 1.048 1.096 1.157 1.299
0.62 0.542 0.568 0.594 0.62 0.646 0.673 0.699 0.726 0.754 0.782 0.81 0.84 0.871 0.903 0.937 0.974 1.015 1.063 1.123 1.265
0.63 509 0.535 561 0.587 0.613 0.64 0.666 0.693 0.721 0.749 0.777 0.807 0.838 0.87 0.904 0.941 0.982 1.03 1.091 1.233
0.64 0.474 0.503 0.529 0.555 0.581 0.608 0.634 0.661 0.689 0.717 0.745 0.775 0.806 0.838 0.872 0.909 0.95 0.998 1.068 1.2
0.65 0.445 0.471 0.497 0.523 0.549 0.576 0.602 0.629 0.657 0.685 0.713 0.743 0.774 0.806 0.84 0.877 0.918 0.966 1.027 1.169
0.66 0.414 0.44 0.466 0.492 0.518 0.545 0.571 0.598 0.626 0.654 0.682 0.712 0.743 0.775 0.809 0.846 0.887 0.935 0.996 1.138
0.67 0.384 0.41 0.436 0.462 0.488 0.515 0.541 0.568 0.596 0.624 0.652 0.682 0.713 0.745 0.779 0.816 0.857 0.905 0.966 1.108
0.68 0.354 0.38 0.406 0.432 0.458 0.485 0.511 0.538 0.566 0.594 0.622 0.652 0.683 0.715 0.749 0.786 0.827 0.875 0.937 1.079
0.69 0.325 0.351 0.377 0.403 0.429 0.456 0.482 0.509 0.537 0.565 0.593 0.623 0.654 0.686 0.72 0.757 0.798 0.846 0.907 1.049
0.7 0.296 0.322 0.348 0.374 0.4 0.427 0.453 0.48 0.508 0.536 0.564 0.594 0.625 0.657 0.691 0.728 0.769 0.817 0.878 1.02
0.71 0.268 0.294 0.32 0.346 0.372 0.399 0.425 0.452 0.48 0.508 0.536 0.566 0.597 0.629 0.663 0.7 0.741 0.789 0.85 0.992
0.72 0.24 0.266 0.292 0.318 0.344 0.371 0.397 0.424 0.452 0.48 0.508 0.538 0.569 0.601 0.635 0.672 0.713 0.761 0.821 0.963
0.73 0.212 0.238 0.264 0.29 0.316 0.343 0.369 0.396 0.424 0.452 0.48 0.51 0.541 0.573 0.607 0.644 0.685 0.733 0.794 0.936
0.74 0.185 0.211 0.237 0.263 0.289 0.316 0.342 0.369 0.397 0.425 0.453 0.483 0.514 0.546 0.58 0.617 0.658 0.706 0.767 0.909
0.75 0.158 0.184 0.21 0.236 0.262 0.289 0.315 0.342 0.37 0.398 0.426 0.456 0.487 0.519 0.553 0.59 0.631 0.679 0.74 0.882
0.76 0.131 0.157 0.183 0.209 0.235 0.262 0.288 0.315 0.343 0.371 0.399 0.429 0.46 0.492 0.526 0.563 0.604 0.652 0.713 0.855
0.77 0.105 0.131 0.157 0.183 0.209 0.236 0.262 0.289 0.317 0.345 0.373 0.403 0.434 0.466 0.5 0.537 0.578 0.626 0.687 0.829
0.78 0.078 0.104 0.13 0.156 0.182 0.209 0.235 0.262 0.29 0.318 0.346 0.376 0.407 0.439 0.473 0.51 0.551 0.599 0.661 0.803
0.79 0.052 0.078 0.104 0.13 0.156 0.183 0.209 0.236 0.264 0.292 0.32 0.35 0.381 0.413 0.447 0.484 0.525 0.573 0.634 0.776
0.8 0.026 0.052 0.078 0.104 0.13 0.157 0.183 0.21 0.238 0.266 0.294 0.324 0.355 0.387 0.421 0.458 0.499 0.547 0.608 0.75
0.81 0 0.026 0.052 0.078 0.104 0.131 0.157 0.184 0.212 0.24 0.268 0.298 0.329 0.361 0.395 0.432 0.473 0.521 0.582 0.724
0.82   0 0.026 0.052 0.078 0.105 0.131 0.158 0.186 0.214 0.242 0.272 0.303 0.335 0.369 0.406 0.447 0.495 0.556 0.698
0.83     0 0.026 0.052 0.079 0.105 0.132 0.16 0.188 0.216 0.246 0.277 0.309 0.343 0.38 0.421 0.469 0.53 0.672
0.84       0 0.026 0.053 0.079 0.106 0.134 0.162 0.19 0.22 0.251 0.283 0.317 0.354 0.395 0.443 0.504 0.645
0.85         0 0.027 0.053 0.08 0.108 0.136 0.164 0.194 0.225 0.257 0.291 0.328 0.369 0.417 0.478 0.62
0.86           0 0.026 0.053 0.081 0.109 0.137 0.167 0.198 0.23 0.264 0.301 0.342 0.39 0.451 0.593
0.87             0 0.027 0.055 0.083 0.111 0.141 0.172 0.204 0.238 0.275 0.316 0.364 0.425 0.567
0.88               0 0.028 0.056 0.084 0.114 0.145 0.177 0.211 0.248 0.289 0.337 0.398 0.54
0.89                 0 0.028 0.056 0.086 0.117 0.149 0.183 0.22 0.261 0.309 0.37 0.512
0.9                   0 0.028 0.058 0.089 0.121 0.155 0.192 0.233 0.281 0.342 0.484
0.91                     0 0.03 0.061 0.093 0.127 0.164 0.205 0.253 0.314 0.456
0.92                       0 0.031 0.063 0.097 0.134 0.175 0.223 0.284 0.426
0.93                         0 0.032 0.066 0.103 0.144 0.192 0.253 0.395
0.94                           0 0.034 0.071 0.112 0.16 0.221 0.363
0.95                             0 0.037 0.079 0.126 0.187 0.328
                                       
Example: Total KW-input of load from wattmeter reading 100 KW at a power factor of 65%. The capacitive KVAR necessary to raise the power factor to 95% is found by multiplying the 100 KW by the factor found un the table (.840). Then 100 KW x 0.840 = 84 KVAR. Use 85 KVAR.                        
                                       

 
Notice:
NEC is a registered trademark of the National Fire Protection Association, Inc.
All NEC references by Electrical Helper are interpretations of the NEC by the Electrical Helper staff and are not NEC statements. The official position of the NEC requires that the NEC be referred to in its entirety.
Important Notice:
Now that we have the power factor capacitor sized we must refer to Article 430.27 in the NEC which states
the conductors shall comply with 460.8 and 460.9
Article 460.8 (A) states the amperage capacity of the capacitor conductors shall not be less than 135 percent of the capacitor rated current in any case and shall not be less than one-third  the ampacity of the motor circuit conductors.

(B) states that an overcurrent device shall be provided in each of the ungrounded conductors for each capacitor bank set to the lowest pratical setting, except if the capacitor bank is connected on the load side of the motor overload protector.

(C) states that a disconnect capable of opening all ungrounded phase conductors to the capacitor bank simultaneously be installed with a rating of not less than 135 percent of the rated current of the capcitor(s) except it is not required when the capcitor(s) is connected on the load side of a motor controller.
Article 460.9 states that the overload current device, shall be set according to the improved power factor and that the effects of the capcitor(s) shall not affect the wire size conductors serving the motor in accordance with Article 430.22